The present invention relates to the signal processing art and, more particularly, to a means for producing a signal which is frequency and phase locked to an input signal.
Systems which frequency of phase lock to a reference input signal are well known, especially in the communications art. Frequency lock systems are commonly employed in tone detect systems which detect for the presence of an audible tone in an input noisy signal. Such systems lock to the frequency of the input tone thus producing an output that is at the same frequency of the tone but which is relatively noise free. Phase lock systems are commonly employed in communication decoding or demodulating schemes. There, for example, a signal in phase lock with a carrier of an input RF signal is created. The recreated carrier may then be used for decoding or demodulting the information signal on the RF signal.
Frequency lock schemes do not necessarily provide phase lock, and a phase lock scheme might not always produce frequency lock, i.e. it might lock an harmonic to the fundamental frequency. In applications wherein both frequency and phase lock are required, the prior art has used an IF discriminator in combination with an IF phase detector. Here, the input RF signal is heterodyned to an IF frequency and passed through a fixed IF filter. The output from the filter feeds to the IF discriminator and IF phase detector which sense frequency and phase change and generate error correction signals. Such IF discriminator phase and frequency lock systems are subject to thermal drift, as well as being expensive and bulky to manufacture.